I. Field of the Invention
This invention generally relates to an X-ray diagnostic apparatus and method utilizing digital radiography, and more particularly to an X-ray diagnostic apparatus utilizing and method digital subtraction angiography.
II. Description of the Prior Art
The development of X-ray diagnostic apparatus and method utilizing digital subtraction angiography has opened the door to substantial progress in the state of the art of X-ray diagnostic apparatus, especially digital subtraction type angiographic diagnostic apparatus.
That is, this diagnostic apparatus employs the so-called "Digital Radiography" or "Digital Fluoroscopy" technique in which time image subtraction and digital image processing are utilized.
As shown in FIG. 1, when time T0 has elapsed after an X-ray contrast medium for a blood vessel was intravenously injected in an upper arm of an object to be examined, such as a patient, a first X-ray projection is performed by an X-ray TV camera (not shown) to photograph the portions of the patient which are not substantially subject to movement such as cervical, cephalic, spiral and abdominal vessel systems as well as leg, aortal, pulmonary artery and peripheral vessel systems. X-ray television picture signals from the X-ray TV camera are A/D converted, and the resultant digital X-ray television picture signals which indicate mask images which contain no contrast medium, are stored in a first digital memory (not shown). When a time interval (T0+T1) has elapsed after the contrast medium had been injected, a second X-ray projection of the portion of the patient to be examined is performed. Thereafter, X-ray projections are intermittently performed at intervals of, for example, one second and the portion of the patient to be examined is projected by X-ray beams and viewed by the X-ray TV camera while the contrast medium interruptedly flows in the portion X-ray television picture signals are produced by the X-ray TV camera and are A/D converted into the digital X-ray television picture signals which are stored in a second digital memory (not shown). The X-ray television picture signals in the first digital memory are subtracted from the X-ray television picture signals in the second digital memory, thereby obtaining subtraction picture signals of an angiogram or a subtraction image showing the flow of contrast medium within blood vessels. The subtraction picture signals are then D/A converted to obtain analog subtraction picture signals, so that subtraction images shown in FIG. 1 are displayed on a CRT monitor. Otherwise, these subtraction images are photographed by a multi-format camera.
Digital radiography has received a great deal of attention since it can be applied to a conventional contrast medium injection method for injecting the contrast medium in an artery using a catheter, and to a vein injection method without using an artery catheter, thus realizing safe, fast and accurate diagnosis.
However, since digital radiography is fundamentally performed by an examination of blood vessels using a contrast medium, and the blood vessels are three-dimensionally distributed, a subtraction image obtained by a unidirectional X-ray projection can only be observed as a two-dimensional image of the three-dimensional object. Consequently, it is very difficult for a doctor to investigate the two-dimensional image as a three-dimensional image. After the first unidirectional X-ray projection is completed, the contrast medium is injected again in the patient. The portion of the patient to be examined is X-ray projected from a different direction from that in the first unidirectional X-ray projection by changing the positioning of the patient or by rotating an X-ray tube and an image intensifier (not shown) with respect to the patient, thereby obtaining a plurality of subtraction images which show the same portion from different angles. The doctor can thus investigate the portion of the patient to be examined by overlaying two different subtraction images in a three-dimensional manner. However, the contrast medium must be injected in the patient every time an X-ray projection is performed from a new angle, so that a total quantity of the injected contrast medium becomes large. As a result, it is difficult to ensure the safety of the patient.